There are a number of instances where it is necessary to determine the amount of current delivered to the distal end of an electrical conductor such as a cable. For example, the above-identified applications, the contents of which are hereby incorporated by reference, disclose an electrosurgical trocar assembly wherein a trocar includes an electrosurgical cutting element connected by a cable to an electrosurgical generator and wherein, in a preferred embodiment, it is desired to shut down the electrosurgical generator when the tip of the trocar penetrates through the wall of the body cavity involved (e.g., the peritoneum). As disclosed in these application, this can be done by sensing the current being delivered by the electrosurgical generator since this delivered current will change when penetration is achieved. Another example of where this is desirable is in connection with r.f. (radio frequency) ablation procedures where there is a need to tightly control delivery of electrosurgical current. The invention will be described below particularly with respect to electrosurgical trocar devices although it is to be understood that the invention is applicable to any situation where there is need to know the amount of a.c. current being delivered to a load at the end of an electrical conductor such as a cable.
Considering the problem to be solved in more detail, when the current being delivered is of high frequency and high voltage as is the current output produced by an electrosurgical generator, a measurement of the total current produced by the generator does not accurately indicate the actual current delivered to the distal end of the electrical connecting cable. The discrepancy or error is due to the distributed capacitance to the current return path of the generator. The current flows through the cable along the entire length thereof and the amount of current flow is determined by the voltage, frequency, distributed capacitance to ground (or return), and cable length. Thus, referring to FIG. 1 wherein an electrosurgical generator is indicated at G, a load impedance (e.g., the impedance of the tissue being operated on by an electrosurgical electrode or cutting element) is indicated at Z.sub.L and a shunt impedance representing the distributed capacitance to ground, i.e., the "leakage" capacitance, is indicated at Z.sub.ca. The generator voltage is V and thus the total current, I.sub.t, can be represented by the equation I.sub.t =V/Z.sub.ca +V/Z.sub.L. Although the current delivered to the load can be derived by measuring V and I.sub.t and then subtracting out the effect of the capacitance, in many cases, and particularly in electrosurgery, the capacitance is unknown and actually varies with the position of the cable in an unpredictable manner, thereby making a simple current measurement at the generator end of the cable inaccurate.